Generally, particle beams are employed in particle beam therapy apparatus designed to cure cancers by irradiating a high energy beam that is obtained by accelerating protons or charged particles, whichever appropriate. The particle beam therapy apparatus is required to be capable of adjusting the trajectory of the beam irradiated from the apparatus in the patient body according to the body part that needs to be irradiated with the beam and the depth of the body part from the body surface of the patient.
A circular accelerator is mostly employed in the particle beam therapy apparatus. A variable energy irradiation method has been proposed for the circular accelerator to adjust the trajectory of an irradiated beam in a body by changing the beam energy to be emitted.
With this method, for example, the beam to be irradiated is separated from circulating beams by means of a charged particle beam extraction device and subsequently the beam is deflected in the opposite direction by means of another charged particle beam extraction device, which may typically be a septum magnet that provides a greater deflection angle.
Besides, there has been proposed a method of generating a beam to be irradiated by controlling the parameters of the high frequency voltage to be applied to the high frequency accelerator cavity of a circular accelerator to change the output current intensity of charged particles to be extracted from a circulation orbit.
Furthermore, for instance, Japanese Patent Application Laid-Open Publication Nos. 2012-22776 and 2011-198748 describe particle beam therapy apparatus that are different from the above-described ones.
Meanwhile, in the circular accelerator and in the particle beam therapy apparatus, the size (including the vertical width and horizontal width) of a beam that is driven to circulate changes depending on the energy of the beam. Particularly, when a beam having a large size and showing a low energy level is to be extracted and output, a sufficient spatial margin (to be referred to as clearance hereinafter) cannot be satisfactorily secured between the component devices of the circular accelerator and the beam to make it impossible to extract and output a beam on a stable basis. The remedy for this defect will apparently be to increase the clearance in order to reduce the loss of beam due to the component devices and stably extract a beam. Then, however, the remedy is accompanied by a problem that the size of the circular accelerator needs to be increased.
Therefore, the object of the present embodiment is to dissolve the above identified problem by providing a circular accelerator that is compact and can extract and output beams on a stable basis over a wide range of energy and a particle beam therapy apparatus including such a circular accelerator.